1. Field of the Invention
The present invention relates to a device for measuring the thickness of non-magnetic layers on magnetic substrates by determining the adhesion of a magnet to the substrate.
2. Description of the Prior Art
Devices of this type are used, for example, to determine the thickness of non-magnetic layers such as varnishes, chrome plating, plastic coatings, etc., on an iron support.
In its most simple form, an adhesion meter for reliably measuring the thickness of a layer by the magnetic process consists of a rod-shaped permanent magnet, which can be removed from the non-magnetic layer by means of a spring. The thinner the non-magnetic layer, the greater the adhesive force. This can be measured according to the spring force needed to remove the permanent magnet. This adhesive force can then be read from a scale (Zeitschrift fur Metallkunde, volume 54, 1963, No. 6, page 339 onwards).
A layer thickness measuring device of the above type is disclosed in German Pat. No. 869 125 and it will be described hereinafter in reference to FIG. 1 of the accompanying drawings.
A balance scale balance 2 is pivotably mounted at a point 7. The point 7 preferably forms the center of gravity of the entire rotating system which will be described hereinafter.
The magnet 3, generally a permanent magnet, is mounted on the leading end of the scale of balance 2. A torque can be exerted on the scale balance 2 by means of a spiral spring 5 acting as the measuring spring. The inner end of the spiral spring 5 is secured to the scale balance 2 and the outer end of the spiral spring 5 is attached to a pivotable graduated disk 4 which is also mounted at point 7.
To carry out a measuring operation the magnet 3 is placed on the non-magnetic layer with the spiral spring 5 in the relaxed state. The thickness of the non-magnetic layer will be determined by measuring the distance between the magnetic support and the contact point of the magnet, i.e., the surface of the non-metallic layer. A pressing pin 6 provided on the other end of the scale balance 2 can be raised so as to pivot the scale balance 2 in a counter-clockwise direction and allow the magnet 3 to exit via an opening in the housing 1 and thus adhere to the measurement point.
By pivoting the scale disk 4 in a clockwise direction the spiral spring 5 is stressed until the magnet 3 is suddenly raised from the surface of the non-magnetic layer. Thus, the adhesion of the magnet 3 to the magnetic layer corresponds to a specific tensional force of the spring 5 and thus to a specific angular position of the scale disc 4. As a result, by corresponding calibration of the device it is possible to directly determine the thickness of the magnetic layer from this angular position.
However, a certain amount of practice and experience is required to use this device and thus the measurements obtained generally depend largely on the way in which the device was used. However, this is extremely unfortunate as subjective influences can so affect the measuring operation that the practicability of such a device is rendered questionable.
As a device of this type is not only used by engineers but also by businessmen, representatives, purchasers, etc., operating errors should, as far as possible, be eliminated and objective measurements sought. However, this can only be achieved by simplifying the method of using the device.
Hitherto, the user had to hold the device in one hand and rotate the scale disc 4 with the index finger or thumb of this hand while taking the measurement. However, it is impossible in this way to hold the device completely still and without vibrating the same on the measuring spot. This is absolutely essential if the thickness of the layer is to be accurately determined.
In the case of an accurate measurement the adhesion of the magnet 3 when it is ripped from the non-magnetic layer is equal to the withdrawal force of the spiral spring 5. However, the equilibrium is unsteady and even slight mechanical influences such as vibrations or knocks could cause the magnet 3 to be raised. An excessive layer thickness would then be indicated and it would be impossible to calculate this error as it would depend on influences which could not be controlled.
These errors are especially marked when the device is used in inaccessible places such as on vertical walls in pipes or above the head on ceilings, etc.
Another essential condition for obtaining an accurate measurement is that the user ceases to rotate the scale disk 4 immediately the magnet 3 is raised from the non-magnetic layer. This is made evident by a clicking sound. However, the actual point at which the rotating movement is ceased depends on the reaction time of the user and thus the scale disk 4 is generally turned a little further than the actual measuring point. As a result, the layer thickness measurement is too low.
Another disadvantage of this device consists in that the user has the impression that he has adjusted the measurement himself and thus the measured layer thickness will be considered as a subjective measurement.